448 Significance HIV is primarily a sexually transmitted disease but little is known about the transmission of viral variants during sexual contact. An understanding of the types of viruses transmitted by sex may make it possible to design vaccines to prevent the transmission of HIV. It has long been held that the genetic material in the virion was RNA only, but it has been shown recently that some transcription of RNA to DNA occurs normally in both SIV and HIV virions. Further is has been demonstrated that virions with DNA transcripts are more efficient at infecting resting lymphocytes than virions without these transcripts. Objectives In this study, we used the rhesus macaque/SIV model to test the 2 hypothesis. One is that reverse transcribed (RT) viruses replicate more efficiently than normal virions. The second is that reverse transcribed (RT) viruses are more efficiently transmitted by sexual contact. Results Eight animals were inoculated intravenously with normal virions and 8 were inoculated with RT-virions. Plasma virus loads during the first eight weeks of infection are currently being assessed. Six animals were inoculated intravaginally with a low dose of SIVmac251 RT, while six animals were inoculated with the same amount of normal virions. Thus far none of the animals have become infected with either of these virus stocks. Thus the presence of DNA transcripts does not seem to have a marked effect on the efficiency at which a virus is transmitted across the vaginal mucosa. Future Directions Studies are continuing in order to determine if there is a subtle effect on transmission that was not noted in this study. KEY WORDS viral variants, HIV mucosal transmission FUNDING NIH Grants AI35545 and RR00169